This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
This has nothing to do with Struts, although it might be possible to compress the stream on-the-fly depending on the compression scheme you want to use. How that would be best accomplished would depend on whether you're using Struts 1 or 2, and would require the use of a custom multipart handler.
It'd be easier to handle it after the file is uploaded, which takes it out of the Struts realm.